Universal Serial Bus (USB) is an industry standard, serial input-output (IO) bus, initially developed in the mid-1990s, that defines the cables, connectors and communications protocols used for connection, communication, and power supply between electronic devices. It is currently developed by the USB Implementers Forum (USB IF). In one configuration, a USB port provides a standard downstream port (SDP) capable of data transfers, that includes four conductors and connector pins: one for bus power (VBUS) at a nominal 5.0 V, two for data signals (Data− and Data+), and one for ground (Ground). USB power, supplied across VBUS and ground, is often sourced from upstream devices or hosts, and is consumed by downstream devices.
Another USB configuration provides a charging port. USB charging ports are divided into two further types: the charging downstream port (CDP), which has the data signals, and the dedicated charging port (DCP), which does not. Dedicated charging ports may be found on USB power adapters that convert utility power or another power supply (e.g., a car's electrical system) to run attached devices and battery packs. In yet another USB configuration, sleep-and-charge USB ports may be used to power or charge peripherals or electronic devices even when the host electronic device or computer is shutdown, so long as power is available to the USB port. Normally, when a computer is shutdown, the USB ports are powered down, preventing peripheral phones and other devices from operating or charging from USB power. Sleep-and-charge USB ports remain powered even when the computer is off. This feature has also been implemented on devices other than computing devices, such as some laptop docking stations, to allow device charging even when no laptop is present.